Sodium sulfite is commonly made by reacting soda ash with sulfur dioxide in an aqueous medium. Sulfur dioxide-containing gas is passed through an aqueous solution of sodium carbonate to form a solution of sodium bisulfite, which is then neutralized, as by addition of further sodium carbonate or of sodium hydroxide to form the desired sodium sulfite. When sodium carbonate is used for neutralization, the solution is boiled to expel evolved carbon dioxide. From the neutralized solution sodium sulfite is obtained by crystallization. If crystallization is carried out below about 35.degree. C., the crystals formed are sodium sulfite heptahydrate (Na.sub.2 SO.sub.3.7H.sub.2 O), which can be transformed into the anhydrous form by heating above about 35.degree. C. At about that temperature the heptahydrate melts incongruently, forming anhydrous sodium sulfite and solution. Alternatively, crystallization of sodium sulfite from the neutralized solution can be conducted at temperatures above 35.degree. C. by evaporating water from the solution, as by boiling it, in which case the crystals formed are anhydrous sodium sulfite. The process involved here, however, is a two step process: formation of sodium bisulfite in the first step, followed by neutralization thereof to form sodium sulfite in the second step. Processes for making sodium sulfite involving the above-described reactions have, for example, been described in U.S. Pat. No. 1,937,944 to Butler; U.S. Pat. No. 2,080,528 to Bowman et al.; U.S. Pat. No. 2,719,075 to Allen; U.S. Pat. No. 2,899,273 to Murphy; and U.S. Pat. No. 3,361,524 and 3,216,793 to Sporman et al. These patents generally are concerned with methods for obtaining anhydrous alkali metal sulfite of relatively high degree of purity, hence include certain further purification steps not of consequence here.
Single-step processes for making anhydrous sodium sulfite are also known and have been described, for example, in U.S. Pat. No. 3,305,307 to Sporman et al, and U.S. Pat. No. 3,213,412 to Carey et al. According to the Sporman et al. patent, solid alkali metal sulfite salt is obtained by contacting an aqueous solution of a suitable alkali metal compound -- such as sodium hydroxide, sodium carbonate, sodium bicarbonate, and the like -- with substantially dry sulfur dioxide-containing gas at temperature sufficiently high that the water introduced with the solution and formed by the reaction of the alkali metal compound with the sulfur dioxide is immediately vaporized. The patent to Carey describes a process wherein an alkali metal salt, such as carbonate of soda, is moistened by contact with a small quantity of water or steam, and the moistened salt is subjected to the action of sulfur dioxide-containing gas. Processes of that kind, however, result in formation of sodium sulfite of relatively low degree of purity, as discussed by Carey et al. in U.S. Pat. No. 3,213,412.
It is an object of the present invention to provide a method for producing anhydrous sodium sulfite by reaction of sodium carbonate with sulfur dioxide in an aqueous medium to obtain crystalline anhydrous sodium sulfite in one-step procedure.
It is a further object of the present invention to provide a method for obtaining substantially concentrated solutions of sodium sulfite of high degree of purity from which sodium sulfite crystals, both anhydrous as well as heptahydrate, may be crystallized in substantially pure form.